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J. J. Butler is with the Laboratory for Terrestrial Physics and C. R. McClain is with the Laboratory for Hydrospheric Processes, both at the National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, Maryland 20771. USA

Abstract

The Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) has made monthly observations of the Moon since 1997. Using 66 monthly measurements, the SeaWiFS calibration team has developed a correction for the instrument’s on-orbit response changes. Concurrently, a lunar irradiance model has been developed by the U.S. Geological Survey (USGS) from extensive Earth-based observations of the Moon. The lunar irradiances measured by SeaWiFS are compared with the USGS model. The comparison shows essentially identical response histories for SeaWiFS, with differences from the model of less than 0.05% per thousand days in the long-term trends. From the SeaWiFS experience we have learned that it is important to view the entire lunar image at a constant phase angle from measurement to measurement and to understand, as best as possible, the size of each lunar image. However, a constant phase angle is not required for using the USGS model. With a long-term satellite lunar data set it is possible to determine instrument changes at a quality level approximating that from the USGS lunar model. However, early in a mission, when the dependence on factors such as phase and libration cannot be adequately determined from satellite measurements alone, the USGS model is critical to an understanding of trends in instruments that use the Moon for calibration. This is the case for SeaWiFS.

References

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a These changes are extrapolations based on the fitted curves in Fig.
9. The launch of SeaWiFS occurred 105 days before the first lunar measurement. The first SeaWiFS image taken when the instrument was turned on occurred 71 days before the first lunar measurement.

a These values have no corrections for the detector response or for the spectral shape of the observed radiation.21 The corresponding wavelengths for the SeaWiFS bands are also listed. They come from system level instrument measurements and include the response of the detectors.7 However, they have no corrections for the spectral shape of the observed radiation.

a The data are the differences of the SeaWiFS lunar measurements from the USGS lunar model. The intercepts are the values of the fitted lines at the time of the first lunar measurement. The standard deviations (about the linear regressions) give a measure of the scatter in the data.

a The data are the differences of the SeaWiFS lunar measurements from the USGS lunar model after correction for the oversampling scatter in the SeaWiFS measurements. The intercepts are the values of the fitted lines at the time of the first lunar measurement. The standard deviations (about the linear regressions) give a measure of the scatter in the data.

a These changes are extrapolations based on the fitted curves in Fig.
9. The launch of SeaWiFS occurred 105 days before the first lunar measurement. The first SeaWiFS image taken when the instrument was turned on occurred 71 days before the first lunar measurement.

a These values have no corrections for the detector response or for the spectral shape of the observed radiation.21 The corresponding wavelengths for the SeaWiFS bands are also listed. They come from system level instrument measurements and include the response of the detectors.7 However, they have no corrections for the spectral shape of the observed radiation.

a The data are the differences of the SeaWiFS lunar measurements from the USGS lunar model. The intercepts are the values of the fitted lines at the time of the first lunar measurement. The standard deviations (about the linear regressions) give a measure of the scatter in the data.

a The data are the differences of the SeaWiFS lunar measurements from the USGS lunar model after correction for the oversampling scatter in the SeaWiFS measurements. The intercepts are the values of the fitted lines at the time of the first lunar measurement. The standard deviations (about the linear regressions) give a measure of the scatter in the data.